(1) Field on the Invention
The present invention relates to a phase-shift photomask (reticle) and a method of fabrication for printing high-resolution images in photoresist, and more particularly to a phase-shift photomask for making closely spaced active areas, plug contacts, and capacitors for high density Dynamic Random Access Memory (DRAM) cells on semiconductor substrates.
(2) Description of the Prior Art
Photolithographic techniques are used to pattern material layers on semiconductor substrates, such as silicon substrates, for making integrated circuits. In these techniques a photoresist layer is coated on a product substrate. The photoresist is then exposed through a photoresist mask, for example a reticle (hereafter referred to simply as a mask), that is stepped across the product substrate (wafer) and a short wavelength radiation, such as ultraviolet (UV) light, is used to expose the photoresist. Typically the mask is composed of a fused silica (commonly referred to as quartz) plate having a patterned opaque layer, such as a thin chromium (Cr) film. After exposing the photoresist on the product wafer through the mask, the photoresist is then developed to define patterns that are used in subsequent processing steps, for example as plasma etch masks, to fabricate the integrated devices. The photoresist mask and etching are used to pattern semiconductor material layers, such as silicon oxide, polysilicon, suicides, and metals on the semiconductor substrate.
As the circuit density continues to increase, such as for ultra-large-scale integration (ULSI), it is necessary to form photoresist patterns having spacings that are less than the exposure wavelength (lambda) of the UV radiation used to expose the photoresist. As is well known in the physical sciences, because of the wave nature of light, diffraction occurs at the edge of the image of the opaque film (Cr film). This diffraction results in the propagation of the light underneath the edge of the patterned Cr film, which limits the resolution when the photoresist layer on the product wafer is exposed through the mask.
To better appreciate this problem, an enlarged top view of a portion of a conventional photomask 10, having alternating transparent areas 4 and opaque areas 2, is shown in FIG. 1A. The opaque areas 2 are formed by patterning a thin Cr layer on the optically transparent quartz plate, also labeled 10. The transparent regions 4 are through the quartz plate where the Cr is removed. Typically the Cr is patterned using a photoresist layer that is exposed using an electron beam (e-beam) exposure tool. Because of the short wavelength associated with the electron beam, it provides a high resolution image that is not achieved using a longer wavelength UV. After developing the e-beam photoresist and patterning the Cr film by etching, the mask is used to expose product wafers using UV light.
When UV light is projected through the mask 10 of FIG. 1A to expose the photoresist on a product substrate, the UV light intensity I varies across the pattern. FIG. 1B shows the variation in the light intensity I across the mask as depicted for 1A-1Axe2x80x2 in FIG. 1A. As shown in FIG. 1B, the light intensity I is a maximum in the transparent regions 4 but does not decrease to zero in the opaque areas 2 because of the constructive interference of the diffracted light at the edge of the opaque areas 2. Since the diffracted light under the opaque areas 2 arrives in phase from adjacent transparent areas 4, the intensity I is not zero but has a finite value Io. Therefore as the image width decreases, it is difficult to expose and develop a high-resolution pattern in the photoresist on the product substrate.
One method of reducing the diffracted light under the narrow opaque regions is described in the reference entitled xe2x80x9cImproved Resolution in Photolithography with a Phase-Shifting Maskxe2x80x9d by M. D. Levenson et al. published in the IEEE Trans. on Elec. Devices, Vol. ED-29, No. 12 December, 1982, page 1828. To better understand using phase-shift masks, one approach is depicted in FIG. 2A. As shown in FIG. 2A, the method involves forming alternating transparent areas 4xe2x80x2 that are 180xc2x0 out of phase with the adjacent areas 4. When UV light is projected through the mask 10 to expose the photoresist on the silicon substrate, the UV light intensity I varies across the pattern through the cross section 2A-2Axe2x80x2, as shown in FIG. 2B. The transmitted UV light intensity I is a maximum in the transparent regions 4 and 4xe2x80x2 but the UV intensity Io is essentially zero under the opaque areas 2 because of the destructive interference of the diffracted light from adjacent transparent areas 4 and 4xe2x80x2 which have a difference in phase of 180xc2x0.
Other methods of making phase-shift photomasks have been reported. In U.S. Pat. No. 5,700,731 to Lin et al. a method is described for making capacitor bottom electrodes using adjacent regions on a phase-shift mask that have a phase shift of 180xc2x0 to expose a photoresist layer. However, Lin et al. do not address the problem of closely spaced openings on a mask that results in poor depth of focus (DOF) due to the diffracted light under the opaque regions. Oi et al. in U.S. Pat. Nos. 5,541,025 and 5,761,075 teach a method of designing layouts for phase-shifting masks to compact the layout of the circuit elements without violating the design ground rule. Oi et al. do not teach a method for making the phase-shift mask. In U.S. Pat. No. 5,786,114 to Hashimoto, a method is described for making halftone attenuated phase-shift masks in which the phase-shift layer at the boundary regions of the mask is removed to reduce the light scattering between adjacent chip areas on the product substrate without requiring an additional opaque boundary area. However, Hashimoto does not address the problem associated with diffracted light between closely spaced transparent openings at the edges of a patterned opaque layer. In U.S. Pat. No. 5,702,847 to Tarumoto et al., a method is described for forming an attenuated phase-shift mask on an optically transmissive substrate and then they describe a method of using the halftone layer on the perimeter of the mask to minimize the transmittance of the radiation. This improves product reliability and turn-around time (TAT).
However, there is still a strong need in the semiconductor industry to fabricate phase-shift masks using a simplified process that is more manufacturing cost effective with greater DOF for improved photoresist resolution. These phase-shift masks are particularly useful for increasing the memory cell density on future DRAM devices having smaller dimensions.
A principal object of the present invention is to provide a phase-shift mask and a method for making this phase-shift mask for exposing photoresist with improved resolution for semiconductor devices having sub-micron dimensions, and more specifically for forming photoresist images having an improved Depth Of Focus (DOF) for active areas, plug areas, and capacitor areas on DRAM devices.
Another object of this invention is to make these phase-shift masks having patterned chrome (opaque) areas and alternate recessed areas on a fused silica plate (commonly referred to as a quartz plate) that form part of the phase-shift mask (reticle).
Still another object of this invention is to use a single photoresist masking step to reduce manufacturing cost.
This invention features a novel phase-shift photomask and a method of fabrication for improving the depth of focus (DOF) for exposing closely spaced patterns in photoresist on a product (semiconductor) substrate. This novel mask is particularly useful for making arrays of closely spaced patterns for the active device areas, polysilicon plugs, and capacitors for memory cell areas on DRAM devices. The phase-shift photomask structure minimizes diffraction at the edge of the opaque patterns resulting in greater DOF and higher resolution when the photoresist on the semiconductor substrate is exposed through the photomask. The method of making these phase-shift masks consists of providing a transparent plate, such as silicon oxide (SiO2), and more specifically a fused silica which is commonly referred to as a quartz plate. An opaque layer is deposited on the transparent plate. Typically the opaque layer is a thin film of chromium (Cr) deposited, for example, by sputter deposition. A photoresist layer is deposited on the opaque layer by spin coating. The photoresist layer is then exposed using an electron beam (e-beam) system to form an array of images. The e-beam dose and electron energy are adjusted during exposure to partially expose (semi-expose) images A while fully exposing the photoresist to form alternate images B. The photoresist is then developed to completely remove the photoresist in the images B thereby exposing the Cr layer in the images B while retaining portions of the photoresist in images A to protect the Cr from etching. The Cr layer is removed from the plate in the regions B. The Cr is removed by wet etching. The photoresist on the plate is plasma etched in oxygen (O2) to remove the remaining photoresist in regions A while retaining the photoresist elsewhere on the photomask. Next, a plasma etch using, for example, CF4 or CHF3, is carried out to selectively etch regions B of the transparent plate, while the Cr in regions A and photoresist protect the remaining transparent plate (photomask) from etching. By the method of this invention, the regions B are recessed to a depth d that provides a phase shift of 180xc2x0 when UV light is later transmitted through the photomask to expose photoresist on semiconductor substrates. The method of making the photomask continues by selectively removing the opaque Cr layer in regions A using wet etching. Then the remaining photoresist is removed to complete the phase-shift mask. Therefore a phase-shift mask is formed using a single photoresist mask in which alternating adjacent regions A and B have an optical path length with a 180xc2x0 phase shift. The diffracted light at the edges of the Cr pattern through regions A and B is 180xc2x0 out of phase and therefore results in destructive interference and minimizes the optical exposure under the Cr pattern resulting in a greater DOF and improved photoresist resolution.
By a second embodiment, the above photomask is subjected to an additional plasma etch which recesses the transparent plate in regions A and B while retaining the difference in depth d between regions A and B. This additional etch results in a better balance in light intensity distribution between the regions A and B.